(1) Field of the Invention
This invention relates to an optical integrated device and, more particularly, to an optical integrated device in which optical parts are contained in the same package.
(2) Description of the Related Art
With the spread of the Internet and cellular telephones, there have been demands for large-capacity transmission lines. Communication networks have come to depend largely on optical fibers which enable high-speed large-capacity transmission, so it is of urgent necessity to provide optical communication networks using techniques, such as wavelength division multiplex (WDM).
FIG. 31 shows the rough structure of an optical transmitter used for WDM transmission. An optical transmitter 50 comprises a semiconductor laser 51, a thermoelectric cooler 52, an optical modulator 53, and a control section 54. An optical fiber 55 to which optical signals are outputted is connected to the optical modulator 53.
The semiconductor laser 51 is a light source from which light is emitted. The thermoelectric cooler 52 controls the temperature of the semiconductor laser 51 so that the optical output and oscillation wavelength of the semiconductor laser 51 will be constant.
The optical modulator 53 is an optical waveguide chip formed by using a substrate made from a material, such as lithium niobate (LiNbO3, hereinafter referred to as LN), having an electro-optic effect. An electrode is located near an optical waveguide. The intensity of light continuously outputted from the semiconductor laser 51 is modulated by electrical signals applied to this electrode to generate optical pulse signals. These optical pulse signals are outputted from the optical fiber 55.
The control section 54 includes a drive circuit for the semiconductor laser 51, a control circuit for the thermoelectric cooler 52 and a drive circuit for the optical modulator 53. The control section 54 sends a drive signal to the semiconductor laser 51 to control it. The control section 54 sends a drive signal to the optical modulator 53 to control it. The control section 54 sends a temperature control signal generated according to a change in the temperature of the semiconductor laser 51 to the thermoelectric cooler 52 to control the temperature of the semiconductor laser 51.
In the conventional optical transmitter 50 having the above structure, usually individual parts including the semiconductor laser 51 and the optical modulator 53 are formed as different optical devices by using optical parts, such as a lens and an optical fiber, and these optical devices are connected by optical fibers and connectors or splices.
Meanwhile, the recent spread of optical communication networks greatly requires that the size and manufacturing costs of optical transmitters and optical receivers should be reduced further. Therefore, the method of connecting individual parts should not be adopted. It is important to locate main parts, such as a semiconductor laser and an optical modulator, in the same package and to make assembly easy.
Conventionally, the following method has been proposed as a technique for locating a semiconductor laser and an optical waveguide chip in the same package. A semiconductor laser and an optical waveguide chip are connected via a polarization maintaining fiber in a package to stabilize the emission of light from the semiconductor laser and to avoid characteristic variations due to a deviation in optical axis (see, for example, Japanese Unexamined Patent Publication No. 2003-295142 (paragraphs [0033]-[0039] and FIG. 4)).
To fabricate an integrated device in which optical parts are contained in one package, the optical parts are adhered to the package with an adhesive. Then the package is put into an oven to heat the package to which the optical parts are adhered. By doing so, the adhesive hardens. Accordingly, if there is a difference in thermal expansion between the package and the optical parts adhered to the package with the adhesive, then a stress may be created due to a difference in contraction, resulting in cracks in one or more optical parts.
Therefore, to fabricate devices in which an optical waveguide chip which is sensitive to an external stress is contained in a package as an optical part, the thermal expansion of the optical waveguide chip must match that of the package. With LN optical waveguide chips, for example, usually stainless steel (SUS, 18Cr-8Ni—Fe) is used as a package material.
In addition, an optical fiber is connected to the optical waveguide chip. Accordingly, in conventional devices, the optical fiber must have a deflection (≦10 μm) in the package with a difference in thermal expansion between the optical waveguide chip and the optical fiber (glass) taken into consideration.
FIG. 32 shows the structure of a conventional optical integrated device. An optical integrated device 60 comprises a package 61, an optical waveguide chip 62, a glass ferrule 63, a metal ferrule 64, and an optical fiber 65.
The optical waveguide chip 62 is adhered and fixed to the package 61. To fix the optical fiber 65, it is inserted into the glass ferrule 63 and the metal ferrule 64. The glass ferrule 63 is adhered and fixed to the optical waveguide chip 62 with an adhesive. The metal ferrule 64 penetrates through a sidewall of the package 61 and is adhered and fixed to the package 61. The metal ferrule 64 is fixed airtightly to the package 61 with solder or the like. The optical fiber 65 inserted into the glass ferrule 63 and the metal ferrule 64 is connected to the optical waveguide chip 62. (After necessary optical parts are located in the package, the package is finally capped and is heated.)
One end of the optical fiber 65 is fixed to the optical waveguide chip 62 and the other end of the optical fiber 65 is fixed to the package 61. As stated above, if they are heated in such a state, the optical fiber 65 is pulled or shrunk by a stress created due to a difference in thermal expansion between the package 61 and the optical fiber 65. Therefore, as can be seen from a figure which shows the optical integrated device 60 from the X direction, the optical fiber 65 in the package 61 has a deflection not greater than about 10 μm. As a result, even if the optical fiber 65 is pulled due to a difference in thermal expansion between the package 61 and the optical fiber 65, a stress will not be applied to the optical fiber 65.
However, a precision fixing technique is necessary for maintaining and fixing a deflection not greater than 10 μm. In addition, a large number of man-hours are taken to perform assembly, and variation occurs depending on fixing methods. Moreover, an excessive deflection leads to an excess insertion loss and a lack of deflection leads to a fracture of the optical fiber due to a difference in thermal expansion.
To fabricate devices in which a semiconductor laser and a thermoelectric cooler, together with an optical waveguide chip, are contained in a package as optical parts, the heat dissipativity of the thermoelectric cooler must be ensured. Accordingly, copper-tungsten (CuW) the heat dissipativity (thermal conductivity) of which is high is often used as a package material.
However, there is a great difference in thermal expansion between a package made from CuW and an optical waveguide chip made from LN (the thermal expansion coefficient of the package (CuW) is 8×10−6 (/° C.) and the thermal expansion coefficient of the optical waveguide chip (LN) is 16.7×10−6(/° C.)). Therefore, if the optical waveguide chip is fixed directly onto the package, an excessive stress is applied due to a difference in thermal expansion and a crack may appear in the optical waveguide chip.
If the optical waveguide chip is fixed onto the package with a soft adhesive, a stress created due to a difference in thermal expansion can be weakened. In this case, however, the soft adhesive must have a measure of thickness, so the optical waveguide chip cannot be fixed firmly onto the package. That is to say, the optical waveguide chip easily moves (vibrates) by vibrations or shocks. This causes a deviation in optical axis between the semiconductor laser and the optical waveguide chip and characteristic variations (insertion loss) are apt to occur.
On the other hand, with the above-mentioned prior art (Japanese Unexamined Patent Publication No. 2003-295142), the semiconductor laser and the optical waveguide chip are connected via the polarization maintaining fiber. If the length of the polarization maintaining fiber connected to the input end of the optical waveguide chip is, for example, about 10 mm, the size of the package is large compared to the case where light condensed by a lens is inputted directly to an optical waveguide.
To reduce the size of the package, it is possible to simply shorten the polarization maintaining fiber. However, if the polarization maintaining fiber is short, even a slight change in position due to, for example, thermal expansion will significantly increase bending R of the polarization maintaining fiber. As a result, the optical loss of the polarization maintaining fiber increases and the reliability of the polarization maintaining fiber deteriorates.
Accordingly, with the optical connection technique using a polarization maintaining fiber, it is difficult to realize miniaturizing a package and maintaining reliability at the same time. In addition, in the above-mentioned prior art, the optical waveguide chip is fixed directly onto the package. Therefore, as stated above, a crack may appear in the optical waveguide chip due to a difference in thermal expansion.